BRACHIOPODA. 
297 
T. simulator, Hall, and some others, possessing a narrow and slightly recurved 
but decidedly elongate brachidium, conveniently constitute a subdivision of 
Cryptonella. But there are other Devonian species in which the loop is far 
more like that of Dielasma, as for example the Terebratula Romingeri, Hall, a 
form widely distributed in the Hamilton fauna of North America; and the 
T. {Cryptonella) lowensis, Calvin, a large, biconvex, and often beautifully pre¬ 
served shell, from the middle Devonian of Fayette and elsewhere in Iowa,* In 
this shell the hinge-plate is constructed as in Cryptonella and is not adherent to 
the bottom of the valve as in Dielasma, though it may be close to it; the crura 
also arise normally from the lateral 
divisions of this plate. With these 
distinctive differences from Dielasma, 
the resemblance to the latter genus in 
the form of the brachidium is striking, 
its descending branches being highly fig. 215 . Terebratula (Cranana) J{omingeri,an\\. 
, , , An outline showing the Dielasma form of the brachidium 
divergent, the ascending branches ab- and the divided i.inge-piate. 
ruptly recurved, making a broad, gentle curvature above ; at the same time this 
recurved band is so very fragile as to be almost invariably destroyed. The entire 
length of the loop, as in Dielasma, and in contradistinction to Cryptonella and 
Eunella, is about one-third that of the brachial valve. This peculiar structural 
variation may be designated by the term CRAN^ENA.f Probably other American 
Devonian species, besides the two mentioned, will be found to belong to this 
group when satisfactory evidence of their internal structure has been obtained. 
Recent observations by Beecher and Schuchert:!; upon the development of 
the brachidium in Dielasma turgida, Hall, of the St. Louis limestone, show that in 
its earliest observed condition, in a shell about 4 mm. in length, it is altogether 
like that of primitive forms of Renssel^ria {R. mutahilis) ; the lateral branches 
uniting by simple coalescence to form a triangular median plate, which is not 
thickened along the line of suture, either below, as in R. mutabilis, or above, as 
* Calvin, Bull. Lab. State University of Iowa, p. 174, pi. iii, fig. 4. 
t To Miss Agnes Crane, of Brighton, England, an associate in the later labors of Dr. Thomas David¬ 
son, and an astute student of the Brachiopoda. 
J Development of the Brachial Supports in Dielasma and Zygospira, op. cit- 1893. 
